1. Field of the Invention
The invention relates generally to a ferroelectric capacitor containing a perovskite structure and a method for fabrication thereof and, more particularly, to a multilayer, titanium containing ferroelectric film having a perovskite structure and a method for fabrication thereof.
2. Description of the Related Art
Modern data processing systems require that a substantial portion of the information stored in their memory be randomly accessible to ensure rapid access to such information. Ferroelectric random access memories (xe2x80x9cFRAMsxe2x80x9d) have been developed for data processing systems operating at high speed. FRAMs exhibit a significant advantage over conventional memories because they are nonvolatile. FRAMs are nonvolatile because they include a ferroelectric capacitor comprising a pair of capacitor plates with a ferroelectric material between them. The ferroelectric material has two different stable polarization states which can be defined with a hysteresis loop depicted by plotting the polarization against the applied voltage.
Recently, ferroelectric materials have been used commercially in the integrated circuit industry. Ferroelectric memories are nonvolatile, programmable with low voltage, such as a voltage of less than 5V (compared to conventional flash memories requiring ,in 18-22V programming voltage), have a fast access time on the order of less than a nanosecond (compared to conventional flash memories with an access time on the order of a microsecond), and have a high robustness and endurance with respect to a virtually unlimited numbers of read and write cycles. The ferroelectric memories also consume low power (less than 1 microampere standby current) and exhibit radiation hardness.
Ferroelectric materials used in integrated circuit applications include perovskite crystal structure ferroelectric dielectric compounds, such as lead zirconium titanate (PbZrxTi1-xO3 or xe2x80x9cPZTxe2x80x9d), barium strontium titanate (xe2x80x9cBSTxe2x80x9d), and lead lanthanum zirconium titanate (xe2x80x9cPLZTxe2x80x9d).
Important aspects of a ferroelectric memory fabrication process are the ability to obtain reliable ferroelectric characteristics without degradation and the formation of a one capacitor/one transistor structure and a multilevel metal structure. Particularly, in case of PZT, its ferroelectric characteristics are directly related to the degree of the perovskite crystallinity after post-deposition annealing. The degree of PZT perovskite crystallinity, and hence its ferroelectric characteristics, are closely related to the ratio of the zirconium to titanium contained in the PZT film.
FIG. 1A shows the relationship between cation content (in atomic %) of lead, zirconium and titanium in a PZT film and distance (in the thickness direction) from a bottom platinum electrode of a capacitor in the PZT film. In FIG. 1A, reference numeral 14 represents the titanium (i.e., Ti ion) content of the PZT film, reference numeral 12 represents the zirconium content of the PZT film and reference numeral 10 represents the lead content of the PZT film. As illustrated in FIG. 1A, the lead content is substantially constant for any distance from the lower electrode (i.e., without regard to the position in the PZT film). However, the content of zirconium and titanium varies significantly with the distance from the lower electrode. Particularly, the zirconium content 12 increases with an increase in distance from the lower electrode of the capacitor (i.e., a larger amount of zirconium is present at an upper portion of the PZT film, away from lower electrode, than at a lower portion of the PZT film, near the lower electrode). On the other hand, the titanium content 14 decreases with increase in the distance from the lower electrode of the capacitor (i.e., a larger amount of titanium is present at a lower portion of the PZT film, near the lower electrode, than at an upper portion of the PZT film, away from the lower electrode).
FIG. 1B shows the relationship between the distance from a lower platinum capacitor electrode and the ratio 18 of the lead content to the sum of the zirconium and titanium content (i.e., [Pb]/[Zr+Ti]) in the PZT film. FIG. 1B also shows the relationship between the distance from the lower electrode and the ratio 16 of the zirconium content to the titanium content in the PZT film (i.e., [Zr]/[Ti]). As can be seen, the lead to zirconium and titanium ratio 18 is substantially constant with distance from the lower electrode, while the zirconium to titanium ratio 16 varies significantly with the distance from the lower electrode.
The variation in the zirconium to titanium ratio with distance from the lower electrode is due to heterogeneous PZT formation and the dependence of the PZT composition on the composition of the capacitor electrodes. The variation in the zirconium to titanium ratio is especially severe when platinum is used as an upper capacitor electrode. Platinum acts as a catalyst in a reduction reaction to further reduce the titanium content in the upper portion of the PZT film near the interface between the upper electrode and the PZT film. The titanium deficiency at the upper portions of the PZT film decreases the perovskite crystallinity of the PZT film and thus decreases FRAM reliability. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.
In accordance with one aspect of the invention there is provided a ferroelectric capacitor, comprising a first electrode, a second electrode and a ferroelectric film between the first electrode and the second electrode, wherein an upper portion of the ferroelectric film contains a higher titanium concentration that a lower portion of the ferroelectric film.
In accordance with another aspect of the invention there is provided a method for fabricating a ferroelectric capacitor, comprising forming a first electrode over a substrate, forming a ferroelectric film over the first electrode, wherein an upper portion of the ferroelectric film contains a higher titanium concentration than a lower portion of the ferroelectric film, and forming a second electrode over the ferroelectric film.
In accordance with yet another aspect of the invention there is provided a method of forming a ferroelectric film having a perovskite structure, comprising forming a ferroelectric layer having a perovskite structure and containing an upper region having a lower titanium concentration than a lower region and supplementing the ferroelectric layer to form a ferroelectric film having a substantially uniform titanium concentration throughout its thickness.